The magnitude of a maximum voltage supported by a drain or source of a MOSFET is determined by a pn junction formed along the boundary of a source or drain. Breakdown, for example, avalanche breakdown, can occur due to an increase in an electric field in a depletion region of the pn junction. The magnitude of the voltage that can be applied to a drain or source in a MOSFET at the onset of breakdown can be determined using conventional techniques. If breakdown occurs at the pn junction formed along the boundary of a source or drain, then current flowing between the drain and source may sharply increase, and this may result in damage to the pn junction and malfunction of the MOSFET.
Since a depletion region at the pn junction is inversely proportional in thickness to the square root of a doping concentration at the pn junction, it is possible to decrease the doping concentration at the pn junction and accordingly increase the thickness of the depletion region and increase the breakdown voltage of the MOSFET.
However, reducing the doping concentration of a drain or source may result in degradation of the MOSFET's electrical characteristics by, for example, increasing an ohmic contact resistance. This problem may be overcome by forming a separate region having a lower doping concentration under the drain or source and optimizing the doping concentration and the size of the separate region, to thereby adjust the breakdown voltage.
Usually in conventional methods for attaining a high voltage MOSFET, a separate underlying region having a lower doping concentration has been formed under a drain or source and the doping concentration and the size of the separate portion have been adjusted. However, the method to adjust the doping concentration of the separate underlying region has made the process for fabricating a MOSFET complicated. Further, when the doping concentration is adjusted by forming a junction in a vertical direction or increasing the junction in a horizontal direction to adjust the size of the underlying region, the process time and/or the size of a device typically increase.